1. Field of the Invention
This invention relates to a method for mooring floating storage vessels, and more particularly to such a method for the permanent mooring of a floating storage vessel for withstanding storms and the like.
2. Description of Prior Art
Mooring systems for floating storage vessels are known in the mooring system art which have a turret provided in a vessel mounted in a well thereof and supported for rotation therein by bearings. Such turret systems can be classified generally as permanently moored systems, in which the turret is anchored via anchor legs to the sea floor or disconnectable moored systems in which the turret may be quickly detached from the anchor legs.
Disconnectable mooring systems find application in bodies of water in which fierce storms occur or in which ice floes are present. Certain disconnectable mooring systems provide a mooring element or buoy which is permanently placed at the offshore field but which may be connected and disconnected from the turret of a production vessel. Thus, in the event that dangerous weather conditions are imminent, the storage vessel may be disconnected from the mooring system and moved to a safe area to wait until the storm or ice floe passes. When the storm conditions pass, the storage vessel is returned to the offshore field and reconnected to the mooring system which has remained in position.
As shown in U.S. Pat. No. 4,604,961 issued Aug. 12, 1986, a well or moon pool is provided between the bow and stern of a vessel. A turret is rotatably mounted in the well at a position adjacent the bottom of the vessel. The mooring system is connected or disconnected from the turret. Once a mooring system is connected to the turret, the vessel is free to move about the turret. A plurality of mooring lines or legs are attached to the turret and extend to the ocean floor. The mooring lines or legs normally comprise chains and wire ropes or cables, and particularly in deep water are of a substantial weight which is exerted against the turret. The turret is mounted in bearings. Frictional forces exerted by the turret against the bearings can be substantial because of the weight of the anchor legs. The anchor lines, particularly when the vessel is anchored in deep water, such as over 200 meters in depth, exert a substantial vertical load on the turret. A number of anchor lines, such as 8 or 10 anchor lines, are spaced at arcuate intervals about the turret with each anchor line exerting a vertical load on a turret.
Heretofore, such as illustrated in U.S. Pat. No. 4,509,448 dated Apr. 9, 1985, a mooring system has been proposed for turret moored drill ships in which a plurality of spaced mooring lines anchored to the sea floor are releasably connected at submersible buoys to the turret of a drill ship. The drill ship has a disconnect/connect system at the submersive buoys so that the drill ship may be rapidly disconnected from its mooring in the event of precarious weather, such as an approaching storm or the like, and moved out of the path of the approaching storm, ice floes, or the like. After the weather has subsided or passed on, the drill ship is returned to its mooring system and reconnected. However, the specific means and steps involved in connecting and disconnecting the vessel turret from the mooring legs is relatively cumbersome and complex.